The present invention relates generally to emissions control systems for reciprocating engines, and more particularly to an emissions control system for increasing selective catalytic reduction (SCR) efficiency through enhanced reactant feed control.
Combustion engines including compression ignition and spark ignition reciprocating engines and gas turbines provide efficient power sources requiring low operating personnel requirements. Combustion engines produce and emit NOx(nitrogen oxides). Control methods to reduce the NOx often increase the fuel consumption of the engines and require large increase in the operating personnel required.
Compression ignition engines, such as diesel engines, provide advantages in fuel economy, but produce and emit both NOx and particulates during normal operation. When primary measures (actions that affect the combustion process itself, such as exhaust gas recirculation and engine timing adjustments) are taken to reduce one, often the other is increased. Thus, combustion conditions selected to reduce pollution from particulates and obtain good fuel economy tend to increase the output of NOx. Current and proposed regulations and legislation present significant challenges to manufacturers to achieve good fuel economy while at the same time reducing the emission levels of particulates and NOx.
In order to meet such requirements or restrictions, a method known as SCR (selective catalytic reduction) has been used for reducing the emission of NOx. The SCR method consists of injecting gaseous ammonia (NH3), ammonia in aqueous solution or aqueous urea, or ammonia supplied from an ammonia generator using a solid source of ammonia such as ammonia carbamate or ammonia carbonate, into the exhaust gas system of the compression ignition engine as a reduction agent. When the temperature of the exhaust gas stream is above a reaction temperature, for example a temperature above 160xc2x0 C. for aqueous urea, the reduction agent undergoes a hydrolysis process and is decomposed into ammonia and CO2. As the exhaust gas stream is passed through the SCR catalyst, the gaseous ammonia reacts with the NOx to reduce the NOx to molecular nitrogen. This reduces or limits the NOx emissions from the compression ignition engine.
The amount of ammonia required at any given time varies as operating conditions of the engine change, and the exhaust gas content includes more or less NOx. It is important that a sufficient amount of ammonia be supplied to treat NOx present in the exhaust gas stream, so that NOx emission standards are achieved. On the other hand, it is wasteful and inefficient to supply ammonia in excess of the amount required to treat the NOx present in the exhaust gas stream.
U.S. Pat. No. 4,403,473 entitled xe2x80x9cAmmonia/Fuel Ratio Control System For Reducing Nitrogen Oxide Emissionsxe2x80x9d, issued Sep. 13, 1983, teaches a method and apparatus for efficiently reducing NOx emissions from an engine. Ammonia is metered to the exhaust gas conduit in a pre-selected proportion to the fuel mass flow rate, but only in response to the temperature of the exhaust gas stream in the reactor being within a pre-selected temperature range.
While the aforementioned U.S. Pat. No. 4,403,473 provides a reasonably reliable method and apparatus for reducing NOx emissions, the method and apparatus do not provide feedback control based on the actual effectiveness of the process. It would be advantageous to control ammonia addition to the exhaust gas stream based on the actual effectiveness of the treatment process.
The present invention is directed to overcoming one or more of the problems as set forth above.
In one aspect of the present invention, an emissions control system for treating an exhaust gas stream with a reduction agent in an exhaust system of an engine is provided with a first sensor for determining at least one operating condition of the engine; and a control unit connected to the sensor for determining a calculated amount of the reduction agent needed to treat the exhaust gas stream. A reduction agent supply source has a first metering means for supplying a first dose of the reduction agent to the exhaust stream in an amount less than the calculated amount of the reduction agent. A reactor has an inlet receiving the exhaust gas stream with the first dose of reduction agent. A second metering means supplies a second dose of the reduction agent to the exhaust stream.
In another aspect of the invention, an engine is provided with a combustion section including a plurality of combustion chambers; a combustion air system supplying combustion air to the combustion chambers, and an exhaust system receiving exhaust gases from the combustion chambers. The exhaust system includes an exhaust manifold and an exhaust conduit for conducting the exhaust gases in an exhaust gas stream from the engine. An emissions control system includes a reduction agent supply source and a reactor having first and second reacting beds in fluid flow communication with the exhaust conduit. A first sensor and a control unit connected to the first sensor determine a calculated amount of the reduction agent needed for treatment of the exhaust gas stream. A first metering means supplies to the exhaust stream a first dose of reduction agent less than the calculated amount of the reduction agent. A second metering means between the reacting beds supplies a second dose of the reduction agent to the exhaust stream. A second sensor determines a characteristic of the exhaust stream, and the control unit is connected to the second sensor for determining the amount of the second dose of reduction agent.
In still another aspect of the invention, a method for increasing the efficiency of an emissions control system for a compression ignition engine capable of producing an exhaust gas stream to be treated by a reduction agent which is mixed with the exhaust gas stream to convert the exhaust gas is provided with steps of: determining a needed amount of the reduction agent to treat the exhaust gas stream; supplying a first dose of the reduction agent to the exhaust gas stream; reacting the exhaust gas stream with the first dose of reduction agent; supplying a second dose of reduction agent to the exhaust gas stream after reacting the exhaust gas stream with the first dose of reduction agent; and reacting the exhaust gas stream with the second dose of reduction agent.
Other aspects and advantages of the present invention will be apparent to those skilled in the art upon reading the following detailed description in connection with the drawing and appended claims.